This invention relates to a process for preparing predispersed fiber compositions and to the compositions prepared thereby. The invention further relates to a process for preparing fiber reinforced rubber or plastic compounds using these predispersed fiber compositions.
2. Description of the Prior Art
The use of discontinuous fibers to reinforce and fill elastomeric and polymeric compounds is well known and is widely practiced in the rubber and plastics industries. Incorporation of discontinuous fiber in rubber or plastic compounds enhances or supplements many of the properties imparted to the final product by the nature of the base compound.
For example, high modulus at low elongation can be obtained by dispersing a sufficient quantity of fiber into a rubber compound. This practice is utilized in the power transmission belt industry in the base stock or undercord of V-belts. The use of fiber in a compound also improves tear resistance, decreases creep or distortion underload, increases the resistance to cuts and punctures, and reduces cut growth. It also improves the green strength of uncured stock to improve handling.
Because of the enhancement of these properties, the use of discontinuous fibers has been adopted for various types of articles associated with the rubber and plastics industries. Discontinuous fibers are used in the fabrication of pH and pond liners, roofing materials, tarpaulins and coated fabrics, power transmission belts and conveyor belts, solid cast tires and components of pneumatic tires, including, but not limited to, tread compounds, undertread or breaker, bead, chipper and sidewall components. The improvement of cut and puncture resistance has given impetus to the tire industry to seek ways to utilize discontinuous fibers in the rubber compounds used to construct tires.
Wider use of discontinuous fibers in various materials has been restricted in part by the difficulty in properly dispersing the individual fibers to obtain their most efficient reinforcing effect. The composition and configuration of the various fibers available also differs widely making some more difficult to be incorporated in the final compound than others. For example, fibers of Kevlar.RTM. aramid pulp are more difficult to incorporate uniformly in a polymer base than a regenerated cellulose due mainly to the differences in configuration and surface area. However, it is highly desirable to take advantage of the unique properties of the aramid fiber by maximizing the degree of dispersibility.
It is difficult to mix discontinuous fibers into massed rubber or plastic polymers and obtain a uniform and homogeneous dispersion of the fibers throughout the polymer because the fibers tend to cling together in bundles and those which are fibrillated tend to become compressed or matted together.
A number of methods for pretreating or predispersing discontinuous fibers have been suggested in an attempt to overcome this problem. For example, in an effort to reduce fiber to fiber interaction, discontinuous fibers have been slurried with a rubber latex or an oil-extended rubber latex, the latex has been coagulated on the fibers, and the coagulated rubber and fiber mixture has been filtered and dried.
Discontinuous fibers have also been pretreated with a minor amount of a plastic polymer in the presence of a lubricant. The lubricant was absorbed by the fibers and enhanced the receptiveness of the fibers to the polymer.
In another method, discontinuous fibers were dusted with a partitioning agent, such as carbon black or clay, to separate the fibers, and then oil was dispersed with and affixed to the fibers and the particles of partitioning agent. Limp fibers, such as aramid and polyester, are coated with a non-elastomeric stiffening agent prior to dusting with the partitioning agent.
In a similar method, aramid pulp is mixed with a reinforcing filler, such as carbon black or silica, and then a soluton of an elastomer in an organic solvent is added to the mixture to produce elastomeric particles, which can then be dried to remove excess solvent.
Glass fibers have been pretreated for use in reinforcing thermoplastic molding compositions by feeding strands of glass fibers through a bath containing an emulsion or solution of a thermoplastic resin in an appropriate solvent. A minor amount of a dispersion aid, such as a high viscosity hydrocarbon lubricant, a plasticizer, or a low molecular weight resin, may be added to the bath in order to facilitate dispersion of the glass fibers during the later melt forming operation. The strands leaving the bath were then passed through an oven to drive off the water or solvent and to fuse the resin. After solidification of the resin, the strands were cut to produce pellets. None of these previous pretreatment methods, however, have been found to be totally satisfactory for preparing a good dispersion of many diverse types of discontinuous fibers in elastomeric and polymeric compounds. Those methods which employ organic solvents are generally considered undesirable because such solvents are volatile and difficult to control and, therefore, present a health hazard. New applications as well as extensions of current applications of composites made from these various types of fibers and polymers would be developed by the rubber and plastics industries if the time and cost of incorporating these discontinuous fibers into an elastomeric or plastic matrix could be reduced. Such new applications would also be developed if the quality of the dispersion of the fibers into the matrix could be improved while utilizing currently available mixing equipment.